Research | Research Group

Surface ferromagnetism in nanoparticles of otherwise nonmagnetic inorganic materials

Nanoparticles of various oxides, nitrides and chalcogenides with no unpaired electrons such as CeO2, Al2O3, MgO, ZnO, In2O3, SnO2, GaN, NbN, CdS and even the ferrroelectric BaTiO3 and superconducting YBa2Cu3O7 were prepared using various chemical methods. Through a very careful and thorough analysis, these nano-materials were shown to exhibit ferromagnetism at room temperature. The unexpected ferromagnetism is confined only to the surface of the nanoparticles and is suggested to be in the anion and/or cation vacancies at the surface. Based on this work, the ferromagnetism of nanoparticles was suggested to be a universal feature, not only of oxides, but also of nitrides and chalcogenides. With its origin in the surface of nanoparticles, it was proposed that such ferromagnetism can coexist with any other functional properties of the bulk form. While incompatible with superconductivity, such ferromagnetism was demonstrated in the normal state of metal oxide and nitride superconductors.


Related Publications

Magnetization reversal in B-site disordered A(Fe0.5Mn0.5)O3 (A=rare earth or Bismuth) Perovskite

Investigations on B-site disordered pervoskite A(Fe0.5Mn0.5)O3 showed interesting magnetic properties. For example, the compound Bi(Fe0.5Mn0.5)O3 exhibits magnetic ordering at high temperature (TN= 270 K). Below TN, the magnetization under low applied field (50 Oe) changes sign below a certain temperature (T*) and remains negative down to low temperature. On the other hand, under negative applied field (-50 Oe) the magnetization becomes positive below T*.


Related Publications

Multiferroics

Experimental identificaiton of intrinsic ferrolectric polarization from pyroelectric current measurements in new magnetoelectric multiferroic materials has been a challenging task particularly in polycrystalline samples due to electric signals arising from leakage current and thermally stimulated free charge carriers which are trapped at the grain boundaries. We have demonstarted a simple DC bias current measurement which can identify instrinsic pyrocurrent due to ferroelectric polarization.As seen below in TbMnO3, the behavior of DC bias current is quite different from that arising from leakage and thermally stimulated charge carries. For details, see ref. 17 in the list of related publications.

We have identified linear magnetoelectric effect in a new family of A-site magnetic spinels.


Related Publications

Fabrication of (Cu,C)Ba2CuO4+δ superconductors

Incorporation of carbon in the form of CO3 group doubles the C- axis length of BaCuO2 to give a modulated structure represented by (Cu,C)Ba2CuO4+δ or (Cu,C)-1201 which forms the first member of the series: (Cu,C)Ba2Can-1CunO2n+2+δ. Increasing substrate temperature drives the completely relaxed (Cu,C)-1201 film to a strained film experiencing a contraction along the in-plane axis. The out-of-plane axis expands simultaneously with the contracting in-plane axis. Superconducting is observed only in strained films with elongated c- axis.


Related Publications

     
     
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